The present invention relates to an ink jet print head and in particular to print heads of the type in which droplets of ink are expelled from a nozzle by rapid heating of a resistive element contained within an ink collecting chamber and disposed next to the nozzle.
The ink collecting chamber and the resistive element are formed within a multi-layer board constructed on a silicon base using well known methods of construction of integrated circuits. The nozzles are constructed, by an electroforming process, in a metal sheet fixed against the multi-layer board, as is described, for example, in European patent application published under No. 401996 in the name of the applicant.
Such print heads are usually provided with a plurality of nozzles and their associated chambers in a vertical row. The print head is moved transversely across a piece of paper and the resistive elements are selectively energised to expel droplets of ink by electrical pulses at predetermined intervals which are limited by a maximum repetition frequency. Characters are thereby printed on the paper.
During rest periods and/or in the interval between two consecutive pulses, it is necessary that the collecting chambers remain completely filled with ink and that the ink forms a stable meniscus in the nozzle. This meniscus should be in equilibrium with the depression in the reservoir of ink in the chamber supply system.
The maximum frequency of operation of such a head is limited by the time taken to replenish the ink in the collecting chamber after the expulsion of a droplet, and by the time taken to damp out the vibrations of the meniscus to return to the equilibrium state.
The time taken to replenish the ink in a chamber is primarily dependent on the hydraulic resistance of the supply duct, while the damping of the meniscus also depends on the lag of the moving body of ink.
One method of modifying the lag of the supply channel in order to improve the performance of a print head-of the type mentioned above, and in particular to reduce the reciprocal interference between the different collecting chambers, is known from European Patent Application No. 314486.
In this the supply channels connecting the reservoir to the collecting chambers each have a constriction. This arrangement, however, has the disadvantage of also increasing the hydraulic resistance of the channel, which reduces the maximum repetition frequency of the expulsion of ink droplets.